1. Field of the Invention
The present invention relates to a method of manufacturing a piezoelectric resonator, for use in an oscillator, for example, and more particularly, to an energy-trap type piezoelectric resonator in which a plurality of excitation electrodes are arranged so as to overlap each other with a piezoelectric body disposed therebetween.
2. Description of the Related Art
One type of conventional energy-trap type piezoelectric resonator has a first excitation electrode and a second excitation electrode arranged on respective major surfaces of a piezoelectric plate such that a first excitation electrode and a second excitation electrode overlap each other with the piezoelectric plate disposed therebetween so as form an excitation portion at the overlapping electrode area. Another conventional resonator is a laminated energy-trap type piezoelectric resonator having at least one internal electrode in which excitation electrodes are arranged on the piezoelectric surface to overlap each other with the internal electrode and a piezoelectric layer disposed therebetween, as disclosed in Japanese Patent Publication (kokoku) No. 7-105686.
In such piezoelectric resonators, the resonance frequency depends upon the thickness of a piezoelectric body. For this reason, in order to produce piezoelectric resonators having different resonance frequencies, a thickness of the piezoelectric bodies is selected according to the desired resonance frequency.
Resonance characteristics of a piezoelectric resonator vary in accordance with the excitation portion defined by the electrode overlapping area of the excitation electrodes. Therefore, first the thickness of a piezoelectric body has to be determined, and then the overlapping area of excitation electrodes is adjusted to a provide an optimum area to achieve excellent resonance characteristics at an arbitrary resonance frequency. The optimum overlapping area of excitation electrodes varies in accordance with the resonance frequency.
Accordingly, in order to realize excellent resonance characteristics at an arbitrary resonance frequency, a piezoelectric body having a thickness corresponding to the resonance frequency must first be provided and subsequently, excitation electrodes must be provided to obtain the an accurate and ideal overlapping area of excitation electrodes in the piezoelectric resonator.
However, during the manufacture of conventional piezoelectric resonators, a plurality of excitation electrodes are usually formed simultaneously on a mother substrate made of a piezoelectric material. This will be described with reference to an example of a method of manufacturing a conventional piezoelectric resonator 70 adapted to vibrate in a thickness shear vibration mode, as depicted in FIG. 13.
When a piezoelectric resonator 70 is manufactured, strip electrodes are arranged with gaps therebetween on the upper surface and the bottom surface of a mother substrate made of a piezoelectric material. The strip electrodes on the upper surface are arranged in order to define an excitation electrode 72a and a lead electrode 72b, depicted in FIG. 13, whereas the strip electrodes on the bottom surface are arranged to define an excitation electrode 72c and a lead electrode 72d.
The strip electrodes on the upper surface of the mother substrate and the strip electrodes on the bottom surface of the same mother substrate overlap each other with a portion of the mother substrate disposed therebetween to thereby have an overlapping length L defined between the excitation electrodes 72a and 72c. The mother substrate is then cut in the direction of a line which connects the excitation electrode 72a and the lead electrode 72b and also in the direction perpendicular to the line, to thereby obtain the piezoelectric resonator 70.
In the piezoelectric resonator 70, the lead electrodes 72b and 72d extend in opposite directions from the excitation portions on a piezoelectric body 71 polarized in the direction of an arrow P. The lead electrode 72b is provided so as to extend to an edge surface 71a, and the lead electrode 72d is provided so as to extend to an edge surface 71b.
As described above, the conventional energy-trap type piezoelectric resonator 70 has generally been manufactured by simultaneously forming a plurality of strip electrodes on respective surfaces of the mother substrate.
As mentioned above, in order to obtain excellent characteristics at a specific resonance frequency, an overlapping area between the excitation electrodes in a piezoelectric resonator, i.e., an overlapping length L between the excitation electrodes 72a and 72c in the piezoelectric resonator 70, must be optimized. Therefore, in the case in which piezoelectric resonators having a variety of frequencies are to be manufactured, after strip electrodes are formed on a mother substrate, for each frequency a separate mask must be provided in order to achieve an optimum electrode overlapping length L for each resonator.
However, the cost to provide a variety of masks corresponding to the respective resonance frequencies is considerably high. Thus, in practice, the frequencies are divided into several frequency ranges and a common mask is used in one frequency range, to thereby form the above-described strip electrodes on a mother substrate.
This process and use of one mask for different resonant frequencies in each of the ranges causes the electrode overlapping length L to be inaccurate and not suitable for some of the resonance frequencies. When an electrode overlapping length L is designated so as to correspond to a specific resonance frequency in a certain frequency range, the electrode overlapping length is not particularly suited for the other resonance frequencies in that range, and therefore, excellent resonance characteristics are only achieved for one of the resonant frequencies in the given range and the other resonant frequencies do not provide excellent resonance characteristics.